Several kinds of DNA testing are available for people who want to find out more about their ancestry. Examples include mitochondrial DNA (mtDNA) analysis to trace a maternal lineage, Y chromosome analysis to trace a paternal lineage, and admixture mapping to determine what percentage of your DNA is African, European, Native American, or Asian. Each method of analysis gives you different information, so the test you choose will depend on what you want to know.
Passing It On
A mother passes her mtDNA to both sons and daughters. However, only daughters can pass it on. Therefore, mtDNA analysis traces only a single direct maternal lineage. Importantly, mtDNA does not change from generation to generation except for rare mutations. Therefore, your mtDNA should be identical to that of your great, great grandmother on your mother’s side. In mtDNA analysis, scientists determine the sequence (the order of the four chemical bases that comprise DNA: A,G,C,T) for the most variable (called “hypervariable” or HV) region. That sequence is then compared to a database of human HV sequences to provide information about where your particular mtDNA originated. It is unlikely that exact matches to your mtDNA will be found in only one ethnic group or country since most mtDNA types are found in a wide geographic area. Two people who have the same type of mtDNA do share a maternal ancestor. However, unless other types of information are available, there is no way of telling if the two people are close relatives or if they shared a maternal ancestor tens or hundreds of generations ago.
From the Father
Y chromosomes are passed only from fathers to their sons and thus they are inherited like last names in many cultures. Most of the Y chromosome does not undergo recombination (DNA shuffling) and hence does not change from generation to generation. However, Y chromosomes contain a large number of small rapidly changing regions called short tandem repeat (STR) or microsatellite loci. Although change in the STR regions is rapid compared to that of other DNA regions, each STR region changes at a rate of once per hundred or thousand generations. Thus, a Y chromosome STR profile based on 12 STR regions would not be expected to change within a few generations. The changes that do occur happen one at a time, so it is easy to connect two lineages that differ by a single change. Therefore, a comparison of a man’s Y chromosome STR profile to a database human Y chromosome STR profiles provides information about the origin of his paternal lineage. Since paternal lineages are shared among people from a wide geographic area, they are unlikely to be found in only one ethnic group or country. When Y chromosome analyses are compared among people with the same last name, they can be used to identify shared paternal lineages that are relatively recent since last names have been in common use for only a few hundred years.
Admixture mapping involves the analysis of genes scattered all over the genome. Particular markers have been identified that are predominately Sub-Saharan African, European, East Asian, or Native American in origin. By evaluating a large number of these markers, the relative proportions of your genetic heritage can be estimated. This approach estimates the relative contributions of all of your ancestors, but it cannot tell you anything about individual ancestors or the precise geographic regions that they came from.
National Geographic Genographic Project
The African American DNA Roots Project has limited resources and a large backlog of samples to analyze. Since we do not charge for our DNA analysis services, we have been overwhelmed by the large number of participants who have sent us samples to be analyzed. Therefore, for the immediate future, we are directing all of our potential participants desiring DNA analysis to the National Geographic Genographic Project (NGGP) in Washington, D.C. The NGGP provides excellent DNA testing services at cost.
We are in the process of merging the Roots Project mtDNA database with that of the NGGP to provide maximum information to participants in both projects. Our primary focus for the near future is to expand our African mtDNA database to learn more about the distribution of genetic diversity among African ethnic groups.